@Bunchy are you still around the forum? Did you have success downgrading?
I've just bought an N10 that turned out to be from T-Mobile (didn't expect this, as it's not a carrier where I live), and am debating whether I need to return it.

The readme that @rubencarneiro mentions (and authored - many thanks!) says the T-Mobile firmware 10.5.8 version, BE88CB, is unsupported.
I presume the whole device is an issue, and that it's probably a bad idea to try to downgrade my T-Mobile phone to the Global BE88AA 10.5.7 firmware?

It may still be possible for me to return this phone and hope to find a Global version.